The project described in this proposal will investigate the role of RNA molecules as effectors in enzymatic catalysis. The model system used will be the aminoacylation by methionyl aminoacyl-tRNA synthetase of a microhelix RNA construct based on the acceptor stem and T-psi-C loop of Escherichia coli tRNA(fMet). In vitro selection methods will be implemented in an attempt to identify one or more DNA aptamers which will act as novel effectors to trigger the protein conformational change which is proposed necessary for a large catalytic rate constant of aminoacylation. The results obtained in such a model system will provide insight into the mechanism of discrimination achieved by aminoacyl-tRNA synthetases. A more complete understanding of this family of enzymes may lead to the development of therapeutic agents which will halt protein synthesis selectively in viral pathogens.